Using assay techniques developed and validated in this laboratory, we are measuring plasma norepinephrine as an indicator of sympathetic neural activity and plasma epinephrine as an indicator of sympathoadrenomedullary activity in hypertension. Research in this area has included several literature analyses; studies of patients with essential hypertension, pheochromocytoma, and normotensive controls at rest; responses to the clonidine suppression test; measurement of disappearance kinetics of injected norepinephrine in patients with hypertension and with orthostatic hypotension; and animal studies involving the renal artery, operant conditioning of blood pressure; and pressor responses to injected norepinephrine and to sympathetic stimulation in pithed rats.